The present invention relates to a multilayer printed wiring board and, more particularly to a multilayer printed wiring board suppressing warping caused by temperature change and an electronic equipment using the multilayer printed wiring board.
A printed wiring board is composed of a composite insulation material formed of fiber woven cloth impregnated with a resin and a circuit pattern of electric conductor formed on the surface of the composite insulation material, and a multilayer printed wiring board is formed by laminating the printed wiring boards in multilayer. E-glass fiber of electric use is generally used for the cloth used for the composite insulation material. Quartz glass fiber or aromatic polyamide fiber is sometimes used for the cloth. A copper film or a copper film having nickel plate on the surface is used for the electric conductor.
The thermal expansion coefficient of the copper film of the electric conductor material is approximately 17xc3x9710xe2x88x926 (1/xc2x0C.), and the thermal expansion coefficient of the composite insulation material made of E-glass fiber and an epoxy group resin is approximately 11xc3x9710xe2x88x926 (1/xc2x0C.). The thermal expansion coefficients of the electric conductor material and the composite insulation material are different from each other as described above. Therefore, the thermal expansion coefficient in the surface direction of the multilayer printed wiring board made of the electric conductor material and the composite insulation material depends on a thickness of the electric conductor forming the circuit pattern and a ratio of the circuit pattern formed of the electric conductor covering the surface of the composite insulation material, that is, a covering ratio.
In general, the circuit patterns formed in the multilayer printed wiring board are not symmetric with respect to the central plane in the thickness direction of the multilayer printed wiring board. In the case where the thicknesses and the covering ratios on the both sides of the electric conductor layers composing the multilayer printed wiring board are asymmetric with respect to the central plane as described above, the thermal expansion coefficients in the surface direction of the electric conductor layers become asymmetric.
Therefore, when the multilayer printed wiring board receives a thermal effect during manufacturing, a warp occurs in the board. If there is a warp in the multilayer printed wiring board, trouble occurs in connecting electronic parts when the electric parts are soldered on the surface of the multilayer printed wiring board because displacement occurs in positional relationship between a junction formed in the part and a junction formed in the multilayer printed wiring board.
In general, the multilayer printed wiring board is formed by that a plurality of printed wiring boards having a circuit pattern on the surface of the composite insulation material are bonded using prepreg by heating at about 170xc2x0 C. Therein, in a case where the thicknesses and the covering ratios on the both sides of the electric conductor layers composing the formed multilayer printed wiring board are asymmetric with respect to the central plane as described above, a warping deformation occurs in the multilayer printed wiring board due to the difference of the thermal expansion coefficients when the multilayer printed wiring board is cooled down to room temperature (20xc2x0 C.) after being heated and bonded at 170xc2x0 C. When electronic parts are soldered on the multilayer printed wiring board, the multilayer printed wiring board is heated up to 170xc2x0 C. or higher. In that time, a warping deformation also occurs in the multilayer printed wiring board due to the difference of the thermal expansion coefficients.
An object of the present invention is to provide a multilayer printed wiring board and an electronic circuit which are small in the warping deformation due to temperature change even if the thicknesses and the covering ratios on the both sides of the electric conductor layer are asymmetric with respect to the central plane. the present invention is characterized by a multilayer printed wiring board formed by laminating composite insulation layers and laminated bodies, the composite insulation layer being made of a cloth and a resin impregnated into the cloth, said laminated body being composed of electric conductor layers formed on a surface of the composite insulation layer, wherein a thermal expansion coefficient of the composite insulation layer having a higher covering ratio by the electric conductor layer is set to a value smaller than a thermal expansion coefficient of the composite insulation layer having a lower covering ratio by the electric conductor layer.
Further, a multilayer printed wiring board in accordance with the present invention is characterized by that a thermal expansion coefficient of the cloth in the composite insulation layer having a higher covering ratio by the electric conductor layer is a value larger than a thermal expansion coefficient of said the in the composite insulation layer having a lower covering ratio by the electric conductor layer.
Further, a multilayer printed wiring board in accordance with the present invention is characterized by that the electric conductor layer comprises an electric power supply layer and a signal layer, and a thermal expansion coefficient of at least one of the composite insulator layers in contact with the electric power supply layer is set to a value smaller than a thermal expansion coefficient of the composite insulator layer in contact with the signal layer.
Further, a multilayer printed wiring board in accordance with the present invention is characterized by that the electric conductor layer comprises an electric power supply layer having a covering ratio of 60 to 80%, preferably 65 to 75% and a signal layer having a covering ratio of 10 to 20%, and a thermal expansion coefficient of at least one of the composite insulator layers in contact with the electric power supply layer is within a range of 8.5 to 9.5xc3x9710xe2x88x926/xc2x0C., and a thermal expansion coefficient of the composite insulator layer in contact with the signal layer is within a range of 10 to 12xc3x9710xe2x88x926/xc2x0C., preferably 10.5 to 11.5xc3x9710xe2x88x926/xc2x0C.
Further, a multilayer printed wiring board in accordance with the present invention is characterized by that the electric conductor layer comprises an electric power supply layer and a signal layer, and the cloth of at least one of the composite insulator layers in contact with the electric power supply layer is made of quartz glass, and the cloth of the composite insulator layer in contact with the signal layer is made of a glass having a thermal expansion coefficient larger than a thermal expansion coefficient of the quartz glass.
As the circuit layers in accordance with the present invention, 4 to 50 layers can be formed.
Further, the present invention is characterized by an electronic equipment comprising electronic parts mounted on a circuit connecting pad formed on a surface of the multilayer printed wiring board described above.
Particularly, in the present invention, a highly productive wiring board can be obtained by that the composite insulator layer is formed by using the same amount of cloth contained in each composite insulator layer and using a material having different thermal expansion coefficient for the cloth. Therefore, it is preferable that quartz glass is used for the composite insulator layer having a larger covering ratio and a glass having a thermal expansion coefficient larger than a thermal expansion coefficient of the quartz glass is used for the composite insulator layer having a smaller covering ratio.
An example of a manufacturing method of the multilayer printed wiring board in accordance with the present invention is as follows.
A sheet of inner prepreg is obtained by impregnating and applying an impregnating epoxy vanish into and to a sheet-shaped base material and by drying it at temperature of room temperature to 170xc2x0 C. Therein, setting of the drying temperature is determined depending on the solvent and the base material used. Circuit patterns are formed on a cured laminated board formed of the obtained prepreg and copper films, and then necessary number of the cured laminated boards having circuit patterns are laminated interposing prepreg sheet between the cured laminated boards, and the laminated cured laminated board is bonded under a condition of 100 to 250xc2x0 C. and 1 to 100 kgf/cm2 to obtain a multi printed wiring board.
In regard to the fiber cloth of the sheet-shaped base material, various kinds of glass fibers such as E-glass fiber, C-glass fiber, A-glass fiber, S-glass fiber, D-glass fiber, YM-31-A glass fiber made from SiO2, Al2O3, and quartz glass fiber can be used as an inorganic fiber. Alamide fiber made from a polymer having an aromatic polyamideimide skeleton can be used as an organic fiber. It is preferable that the fiber cloth is contained 20 to 40 volumetric % to the insulator layer and has a diameter of 10 to 20 xcexcm.
It is preferable that one layer of the insulator layer is composed of a plurality of the fiber cloth layers, and the insulator layers are laminated alternatively in a thinner layer and a thicker layer through the circuit conductor layer, and line width of a signal layer and a matching layer is smaller than 100 xcexcm.
It is preferable that thickness of the insulator layer is smaller than 250 xcexcm, and number of the fiber cloth layers per one layer of the insulator layer is 2 to 5. Particularly, it is preferable that one layer of the thinner insulator layer is composed of 2 layers of the fiber cloth layers and has a thickness smaller than 100 xcexcm and one layer of the thicker insulator layer is composed of 3 layers of the fiber cloth layers and has a thickness smaller than 150 xcexcm.
In the present invention, in a computer connecting structure in which pins provided in a ceramic multilayer wiring board mounting a plurality of semiconductor elements are inserted into and fixed to through holes provided in a multilayer printed wiring board, and terminals of the multilayer printed wiring board are electrically connected to a multi-conductor connector provided in a back board, at least one of the multilayer printed wiring board and the back board is formed of the above-mentioned board alternatively laminated the insulator layer the fiber cloth impregnated with a resin and the circuit conductor layer, and the insulator layer has a specific dielectric constant in 1 MHz of 3.0 to 3.5, a flame retardancy in UL94 of V-0 and a thermal expansion coefficient of 5.0xc3x9710xe2x88x925 to 8.0xc3x9710xe2x88x925/xc2x0C., and a line width of the circuit signal layer is smaller than 100 xcexcm.
In addition to the above, the multilayer printed wiring board in accordance with the present invention can be used for a printed board of an electronic unit of a cellular phone, personal computer or a ECU of a vehicle.